La Liga outfit Espanyol announced on Saturday that they had agreed to sell the 24-year-old to the Serie A side, and the deal has now been ratified.
The agreement is believed to be worth around €5 million and sees Lopez become Napoli's fifth acquisition of the transfer window.
The defensive midfielder will likely fill the void left by Valon Behrami, who departed the Stadio San Paolo to join Bundesliga side Hamburg.
Lopez told Napoli’s official website: "I'm happy to be at Napoli, an important club with a wonderful audience."
A product of Espanyol's academy system, Lopez made 41 appearances in all competitions for the Catalan club, scoring three goals.
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